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Trimming the Mainsail with Rod Davis Part #1
The mainsail is the hardest and most fun sail to trim. You cannot change a mainsail as you can jibs. The one main has to look full and powerful in three knots of breeze and become flat and efficient in twenty. The main on a masthead boat is more like an aeroplane flap or an extension of the wing. This flap has a great effect on the lift versus drag of the whole sail plan. On a fractional rig the main makes up a large part of the driving force and is part of the wing itself not just an extension.
The mainsail trimmer has more controls at his disposal than anyone else on board. The mainsheet, traveller, Cunningham, outhaul, mast bend, and reefs all help to give this sail the necessary range. The main can also act like a rudder balancing the helm and keeping the boat tracking.
Mast bend is probably the most effective way to change the shape of the mainsail while sailing, so it is important to have the right luff curve of the sail to fit your mast. Luff curve is the round in the front of the sail which matches the bend in the mast and offsets the broad seaming (shaping) in the panels. At Lidgard Sails, computers match luff curve, cloth stretch, broad seaming and mast bend to make sure your main is easy to trim and has maximum range. Bending the mast will flatten the main and allow the leech to open off. On boats with an adjustable backstay you have the ability to bend the mast. Even the boom vang can be used to get bend in the bottom of the mast when the mast is stepped on the deck.
The outhaul controls the foot and up to the bottom quarter of the mainsail. When the outhaul is eased a small shelf will appear next to the boom. This will add considerable depth to the bottom of the sail for sailing in light air or off the wind. often the outhaul adjustment will have an effect on how the boat is pointing. Easing the outhaul adding fullness to the main will cause the main to have more lift (and more drag) creating more weather helm. The end result is that the boat will point higher and go a little slower through the water. In windier conditions the main has got enough lift due to the amount of air passing by the sail. The boat has weather helm, by tightening the outhaul the main is flattened the drag is reduced and the boat will go faster.
A flattening reef is a cringle placed about eight inches above the clew ring. This is really an extension of the outhaul and continues to flatten the sail. The area it takes out is insignificant. Flattening reefs are not used as much today and hardly ever used on boats with masts that can be bent.
The Cunningham (named after Briggs Cunningham who invented the device in 1958 on the 12 metre "Columbia") is used to adjust the draft location through luff tension. The tighter the luff tension the further forward the draft will come. The main should fit perfectly between the black bands when the halyard is just tight.
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